One of the early events after fertilization of sea urchin eggs is the outgrowth of more than 105 microvilli, which contain actin filaments in the form of a bundle (Burgess and Schroeder. 1977. J. Cell Biol. 74, 1032). The detailed substructure of these bundles has been determined by image processing (Spudich and Amos. 1979. J. Mol. Biol. 129, 319), and reconstitution of the bundles from actin and one other protein has been achieved by Bryan and Kane (1981. J. Mol. Biol. 125, 207). Begg and Rebhun (1979. J. Cell Biol. 83, 241) reported that cortices from unfertilized eggs do not contain actin filaments, but that filaments appear upon raising the pH to 7.5. This transformation can be readily studied since eggs can be obtained in large quantities and about 20% of the actin of the unfertilized egg is associated with the cortical layer (Spudich and Spudich. 1979. J. Cell Biol. 82, 212). We are analyzing the actin of unfertilized and fertilized eggs biochemically and structurally. Accessory components that modify the actin by binding to it or enzymatically altering it are being purified and characterized. In addition, we will work to establish an assay for the elongation of microvilli in permeabilized cells. Our objective will be to extract components essential to this elongation and identify them byreconstitution experiments. Other experiments will be designed to determine the direction of growth of the actin filaments in the sea urchin egg microvilli.